The present invention relates to a switching circuit of a switched-mode power supply device. More specifically, the present invention relates to a semiconductor substrate, a semiconductor chip and a semiconductor device used in a switching circuit.
There is an existing technology of reducing conduction loss by shortening a conducting period of a body diode being parasitic to a low-side MOSFET of a synchronous rectifying device in a feedback period in which a high-side MOSFET of a main switching device is off (for example, Japanese Patent Application Laid-Open Publication No. 2004-312913 (Patent Document 1)).
According to Patent Document 1, not to cause large through current to occur when a high-side MOSFET of a main switching device and a low-side MOSFET of a synchronous rectifying device are turned on at the same time, the high-side MOSFET and the low-side MOSFET are alternately turned on and off with dead time provided to gate voltage signals.
According to Patent Document 1, by control for making the dead time of the turning on and off the MOSFETs, a conducting period of a body diode parasitic to the low-side MOSFET in a feedback period in which the high-side MOSFET is on is shortened, thereby reducing conduction loss.
As a voltage to be applied to the body diode of the low-side MOSFET is switched from a forward voltage to a reverse voltage upon turning on the high-side MOSFET, reverse current from a cathode to an anode, that is, recovery current flows in the body diode for a moment, thereby causing recovery loss to occur.
To reduce the recovery loss, it has been known that crossing a high-side MOSFET and a low-side MOS is effective (for example, Japanese Patent Application Laid-Open Publication No. 2007-14059 (Patent Document 2)). However, if the switching timing is crossed too much, there is a problem that large through current flows.
According to Patent Document 1, there is a limitation not to flow the through current at all, and thus the switching cannot be crossed for reducing the recovery loss. According to Patent Document 2, with preventing large through current from flowing, the switching of the high-side MOSFET and the low-side MOSFET is crossed, thereby reducing the recovery loss.
The switching circuit of Patent Document 2 includes first current detecting means which detects current flowing in the low-side MOSFET and second current detecting means which detects current flowing in the body diode of the low-side MOSFET, in which dead time is set to make both through current detected by the first current detecting means and recovery current of the body diode detected by the second current detecting means small, and gate signals to the high-side MOSFET and the low-side MOSFET are outputted. In this manner, as well as reducing recovery loss by flowing the through current, large through current is prevented from flowing.
However, in a generally used MOSFET, a source electrode is in contact with both a p-type body region and an n+-type source region, and thus it is difficult to independently detect operating current of the MOSFET and current flowing in the body diode.
According to Japanese Patent Application Laid-Open Publication No. 5-75131 (Patent Document 3), there is a problem that operating current of a MOSFET cannot be independently detected while it is possible to independently detect current flowing in a body diode.
To solve this problem, according to Japanese Patent Application Laid-Open Publication No. 2006-310473 (Patent Document 4), structures of a sensing MOSFET and a sensing diode capable of independently detecting operating current of a MOSFET and current flowing in a parasitic diode are suggested.